1. Field of the Invention
The present invention relates in general to an apparatus for preventing excessive slipping of wheels of a motor vehicle upon acceleration or braking of the vehicle.
2. Discussion of the Related Art
A wheel of a motor vehicle will slip on a road surface to an excessive extent when a drive force transmitted to the wheel upon acceleration of the vehicle is excessive in relation to a force of friction between the wheel tire and the road surface, or when a brake force applied to the wheel upon braking of the vehicle is excessive in relation to the force of friction between the tire and the road surface. The acceleration or braking of the vehicle is effected in dependence on the force of friction between the wheel tire and the road surface. The force of friction is maximized when the ratio of slip of the wheel relative to the road surface (namely, the slip amount of the wheel divided by the vehicle speed) is optimum. Accordingly, the acceleration or braking of the vehicle need to be effected with the slip ratio or slip amount of the wheel maintained in an optimum range, so that the vehicle is accelerated or braked in a satisfactory fashion.
In view of the above need, a brake system of a motor vehicle known in the art is adapted to effect a traction control of a wheel drive force and/or an anti-lock control of a wheel brake force. In the wheel traction control, a brake force is applied to a driving wheel of the vehicle to reduce an effective drive force generated by the driving wheel, for thereby avoiding excessive slipping of the driving wheel upon acceleration of the vehicle. In the anti-lock control, the brake force applied to a wheel of the vehicle is reduced so as to avoid excessive slipping of the wheel upon braking of the vehicle.
The brake force applied to the vehicle wheel as described above is regulated in various manners. An example of such manners of regulating the brake force is disclosed in JP-A 2-310161. According to this publication the brake force is regulated on the basis of an amount of change in the rotating speed of the wheel, more specifically, on the basis of only the amount of change in the wheel speed, or on the basis of not only the wheel speed change amount but also another parameter such as the wheel speed per se or the vehicle speed. It is noted that an acceleration value of the wheel, which is the amount of change in the wheel speed per unit time, is considered to be a kind of the wheel speed change amount. In the present application, however, the amount of change in the wheel speed will not be referred to as "wheel acceleration", since the amount of change in the wheel speed is treated herein as a parameter which is not necessarily an amount of change per unit time but a parameter obtained within a given time duration.
The slip amount or ratio of the vehicle wheel is calculated from the vehicle speed and the wheel speed. An amount of increase or decrease in the wheel slip amount or ratio can be obtained from the amount of change in the wheel speed. Therefore, the slip amount or ratio of the wheel can be maintained within the optimum range when the brake force applied to the wheel is suitably controlled on the basis of either the wheel speed, vehicle speed and wheel speed change amount in combination, or the wheel speed change amount alone.
Conventionally, the wheel speed change amount is detected by a suitable wheel speed sensor as an amount of change in the actual wheel speed. However, the wheel speed change amount thus obtained includes not only an amount of change in the wheel speed which arises from the slipping of the wheel on the road surface, but also an amount of change in the actual vehicle speed. Although the wheel speed change amount which does not include the vehicle speed change amount accurately represents a change in the slip amount or ratio of the wheel, the conventional arrangement uses the wheel speed change amount including the vehicle speed change amount, for regulating the brake force applied to the wheel to control the wheel slip amount or ratio.